Cavity enhanced laser based isotopic gas analyzer

ABSTRACT

Systems and methods for measuring the isotope ratio of one or more trace gases and/or components of gas mixtures such as different gas species present in a gas mixture. The system includes a resonant optical cavity having two or more mirrors and containing a gas, the cavity having a free spectral range that equals the difference between frequencies of two measured absorption lines of different gas species in the gas, or of two different isotopes, divided onto an integer number. The system also includes a continuous-wave tunable laser optically coupled with the resonant optical cavity, and a detector system for measuring an absorption of laser light by the gas in the cavity. The detector system includes one of a photo-detector configured to measure an intensity of the intra-cavity light or both a photo-acoustic sensor configured to measure photo-acoustic waves generated in the cavity and a photo-detector configured to measure an intensity of the intra-cavity light.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation application of U.S. Non-provisionalapplication Ser. No. 14/156,842, filed Jan. 16, 2014, which claims thebenefit of, and priority to, Ser. No. 13/538,620, filed Jun. 29, 2012,which claims the benefit of, and priority to, U.S. provisional Patentapplication No. 61/524,911, filed Aug. 18, 2011, the contents of whichare each hereby incorporated by reference.

BACKGROUND

The present invention relates generally to trace gas detection and morespecifically to cavity enhanced absorption spectroscopy systems andmethods.

Optical absorption spectroscopy involves passing radiation through asample, e.g., an analyte, an inferring properties of the sample frommeasurements performed on the radiation. For example, trace gasdetection can be spectroscopically performed by taking measurements todetect the presence or absence of spectral absorption linescorresponding to the gas species of interest. Spectroscopic analysis ofisotopes can also be performed. However, because the integral lineintensities of absorption gas lines are sensitive to the gastemperature, and the pressure broadening of those lines is sensitive tothe gas pressure and the gas composition, measurements of the isotopicratio with high accuracy require measuring of the analyzed gastemperature and pressure with high accuracy, and measuring of thecomposition of major components of the analyzed gas. Moreover, because ameasurement of the isotopic ratio very often requires working at low gaspressure, when gas absorption lines are narrow and their mutualoverlapping decreased, it can be very hard to precisely measure theintegral intensities of the absorption lines. Such measurements of theintegral intensities require very precise measurements of laserfrequency.

Accordingly it is desirable to provide improved spectroscopy systems andmethods for measuring gas species and/or isotopes.

SUMMARY

The present invention provides systems and methods for measuring theisotope ratio of one or more trace gases and/or components of gasmixtures such as different gas species present in a gas mixture.

Embodiments of the present invention provide systems and devices fordetecting the isotopic ratio of the analyzed gas with high accuracyusing a resonance optical cavity, which contains a gas mixture to beanalyzed, a laser coupled to the cavity, and a light sensitive detector.The optical cavity can include any type of cavity with two or morecavity mirrors, including a linear or a ring cavity. A laser that iscapable of being frequency-scanned is coupled to the cavity though oneof the cavity mirrors (i.e., the cavity coupling mirror). A detectionmethod can be based on any of a variety of cavity enhanced opticalspectroscopy (CEOS) methods, for example, cavity ring-down spectroscopy(CRDS) methods, cavity phase shift spectroscopy methods, cavity enhancedabsorption spectroscopy (CEAS) methods, or cavity enhancedphoto-acoustic spectroscopy (CE-PAS) methods (see, e.g., U.S. patentapplication Ser. No. 12/660,614, (US Published Patent application2011-0214479 A1, now U.S. Pat. No. 8,327,686) filed on Mar. 2, 2010,entitled “METHOD AND APPARATUS FOR THE PHOTO-ACOUSTIC IDENTIFICATION ANDQUANTIFICATION OF ANALYTE SPECIES IN A GASEOUS OR LIQUID MEDIUM”, thecontents of which are hereby incorporated by reference).

Because the integral line intensities of gas absorption lines aresensitive to the gas temperature, and the pressure broadening of thoselines is sensitive to the gas pressure and the gas composition,measurements of the isotopic ratio with high accuracy require measuringof the analyzed gas temperature and pressure with high accuracy, andmeasuring of the composition of major components of the analyzed gas.Moreover, because a measurement of the isotopic ratio very oftenrequires working at low gas pressure, when gas absorption lines arenarrow and their mutual overlapping decreased, it can be very hard toprecisely measure the integral intensities of the absorption lines. Suchmeasurements of the integral intensities require very precisemeasurements of laser frequency. The task is simplified if themeasurements of the peak intensities provide the required accuracy.

The approach of one embodiment is based on the fact that absorptionlines of different isotopes may have similar temperature dependences andpressure broadening coefficients, particularly isotopes having closequantum numbers as shown in FIGS. 1 and 2. So, if two lines of differentisotopes with close quantum numbers are chosen, then instead ofmeasuring the ratio of integral intensities of the corresponding linestheir peak intensities are measured. However, if it is still necessaryto measure their integral intensities, this can be done by a synchronousscanning the cavity modes through both spectroscopic features. Thesynchronous measurements of the integral intensities of both lines willbe more accurate because two lines have close pressure broadeningparameters.

Embodiments of the present invention allow for replacing the morecomplex measurements of the line area (i.e., integral intensity) withsimpler measurements of the peak height, which is possible if the linesof two isotopologues react to the ambient condition changes in the sameor a similar way. Using close or similar quantum numbers will also helpif the integral line intensities are measured and compared.

According to an embodiment, a gas analyzer system is provided formeasuring a concentration of two or more components in a gas mixture.The system typically includes a resonant optical cavity having two ormore mirrors and containing a gas having chemical species to bemeasured, the cavity having a free spectral range that equals thedifference between frequencies of two measured absorption lines ofdifferent gas species divided onto an integer number. The system alsotypically includes a continuous-wave tunable laser optically coupledwith the resonant optical cavity, and a detector system for measuring anabsorption of laser light by the gas in the cavity. In certain aspects,the gas analyzer system also includes a temperature sensor for measuringa temperature of the gas in the cavity, and a pressure sensor formeasuring a pressure of the gas in the cavity. In certain aspects, thedetector system includes one of a photo-detector configured to measurean intensity of the intra-cavity light or both a photo-acoustic sensorconfigured to measure photo-acoustic waves generated in the cavity and aphoto-detector configured to measure an intensity of the intra-cavitylight.

According to another embodiment, a gas analyzer system is provided formeasuring an isotopic ratio of a gas. The system typically includes aresonant optical cavity having two or more mirrors and containing a gashaving a chemical species to be measured, the cavity having a freespectral range that equals the difference between frequencies of themeasured absorption lines of two different isotopes divided onto aninteger number, a continuous-wave tunable laser optically coupled to theresonant optical cavity, and a detector system for measuring anabsorption of laser light by the gas in the cavity. In certain aspects,the gas analyzer system also includes a temperature sensor for measuringa temperature of the gas in the cavity, and a pressure sensor formeasuring a pressure of the gas in the cavity. In certain aspects, thedetector system includes one of a photo-detector configured to measurean intensity of the intra-cavity light or both a photo-acoustic sensorconfigured to measure photo-acoustic waves generated in the cavity and aphoto-detector configured to measure an intensity of the intra-cavitylight.

According to yet another embodiment, a system for measuring the isotopicratio of a gas is provided. The system typically includes a resonantoptical cavity containing a gas with chemical species to be measured andhaving a free spectral range equal to the difference between frequenciesof the measured absorption lines of different isotopes divided onto aninteger number, and a continuous-wave tunable coherent light source,such as a laser, optically coupled to the resonant optical cavity. Thesystem also typically includes a detector for measuring an absorptioncoefficient. In one embodiment, the detector includes a photo-detectorfor measuring the intensity of the intra-cavity light. The system alsotypically includes a temperature sensor for measuring the temperature ofthe analyzed gas, and a pressure sensor for measuring the pressure ofthe analyzed gas.

According to a further embodiment, a method is provided for performingan absorption measurement. The method can be implemented in a systemdescribed above, or in a different system. The method typically includesselecting absorption lines of different isotopes having equal or closequantum numbers, for example dn=−2, 0, +2, or dn=−2, −1, 0, +1, +2, ordn=−1, 0, +1, and tuning a cavity mode to a first wavelengthcorresponding to an absorption line of one of the isotopes. The methodalso typically includes generating light having the first wavelengthcorresponding to an absorption line of one of the isotopes, measuring afirst signal representing an absorption coefficient for the firstwavelength, e.g., measuring a signal corresponding to the intra-cavityoptical power at the first wavelength, tuning a cavity mode to a secondwavelength corresponding to an absorption line of the second isotope,generating light having the second wavelength corresponding to anabsorption line of the second isotope and measuring a second signalrepresenting an absorption coefficient for the second wavelength, e.g.,measuring a signal corresponding to the intra-cavity optical power atthe second wavelength. The method also typically includes calculating anisotope ratio based on the first and second measured signals. In certainaspects, a baseline is defined or determined by measuring an absorptioncoefficient at a wavelength that does not correspond with an absorptionline of any of the isotopes being measured or analyzed.

According to another embodiment, a method is provided for performing anabsorption measurement. The method can be implemented in a systemdescribed above, or in a different system. The method typically includestuning a cavity mode to a first wavelength corresponding to anabsorption line of a first one of at least two different isotopes thathave equal or close quantum numbers, generating light comprising thefirst wavelength corresponding to an absorption line of the isotopes,and measuring a signal corresponding to an absorption coefficient at thefirst wavelength. The method also typically includes tuning a cavitymode to a second wavelength corresponding to an absorption line of asecond isotope, generating light comprising the second wavelengthcorresponding to an absorption line of the second isotope, and measuringa signal corresponding to an absorption coefficient at the secondwavelength. The method also typically includes calculating the isotoperatio based on two measured signals. In certain aspects, a baseline isdefined or determined by measuring an absorption coefficient at awavelength that does not correspond with an absorption line of any ofthe isotopes being measured or analyzed.

According to yet a further embodiment, a method is provided forperforming an absorption measurement. The method can be implemented in asystem described above, or in a different system. The method typicallyincludes selecting absorption lines of different isotopes that haveequal or close quantum numbers, e.g., dn=2, 0, +2, or dn=−2, −1, 0, +1,+2, or dn=−1, 0, +1, and selecting or adjusting the cavity length suchthat the difference between frequencies of the measured absorption linesof different isotopes is a product of an integer number and the cavityfree spectral range. The method also typically includes tuning a cavitymode to a wavelength corresponding to an absorption line of one of theisotopes, generating light having a first wavelength corresponding to anabsorption line of one of the isotopes, and measuring a signalrepresenting an absorption coefficient for the first wavelength, e.g.,measuring a photo-acoustic signal and/or measuring a signalcorresponding to the intra-cavity optical power at the first wavelength.The method also typically includes generating light having a secondwavelength corresponding to an absorption line of the second isotope,and measuring a signal representing an absorption coefficient for thesecond wavelength, e.g., measuring a signal corresponding to theintra-cavity optical power at the second wavelength. The method alsotypically includes calculating an isotope ratio based on the measuredsignals. In certain aspects, a baseline is defined or determined bymeasuring an absorption coefficient at a wavelength that does notcorrespond with an absorption line of any of the isotopes being measuredor analyzed.

According to a further embodiment, a method is provided for performingan absorption measurement. The method can be implemented in a systemdescribed above, or in a different system. The method typically includesselecting absorption lines of different isotopes that have equal orclose quantum numbers, e.g., dn=2, 0, +2, or dn=−2, −1, 0, +1, +2, ordn=−1, 0, +1, and selecting or adjusting a cavity length such that thedifference between frequencies of the measured absorption lines ofdifferent isotopes is a product of an integer number and the cavity freespectral range. The method also typically includes tuning a cavity modeto a wavelength corresponding to an absorption line of one of theisotopes, generating light having a first wavelength corresponding to anabsorption line of a first one of the isotopes, and generating lighthaving a second wavelength corresponding to an absorption line of asecond isotope. The method also typically includes measuring signalsrepresenting absorption coefficients for the first and secondwavelengths, e.g., measuring photo-acoustic signals and/or measuringsignals corresponding to the intra-cavity optical power at the first andsecond wavelengths, and calculating an isotope ratio based on themeasured signals. In certain aspects, a baseline is defined ordetermined by measuring an absorption coefficient at a wavelength thatdoes not correspond with an absorption line of any of the isotopes beingmeasured or analyzed.

In certain aspects, measurements of absorption of the gas mixture aremade not only at wavelengths corresponding to absorption lines ofisotopes, but at other wavelengths, for example, where there is noabsorption. This can be useful to determine or define a baseline. Forexample, measuring only two peak intensities for two isotopologue linesmay not be sufficient, and at least one more measurement in the areathat does not belong to any of the two absorption lines may be needed.Such a measurement gives the information about the baseline.

Reference to the remaining portions of the specification, including thedrawings and claims, will realize other features and advantages of thepresent invention. Further features and advantages of the presentinvention, as well as the structure and operation of various embodimentsof the present invention, are described in detail below with respect tothe accompanying drawings. In the drawings, like reference numbersindicate identical or functionally similar elements.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates temperature dependences of the R (6), R (36), R (8),R (10), R (12), R (14), R (16), R (18), and R (36) lines of 12C16O16Oand the R(12) line of 13C16O16O for the (3,00,1)-(0,00,0) combinationband of carbon dioxide. The graph shows that the temperature dependencesof the integral intensities of lines with different quantum numbers arequite different.

FIG. 2 illustrates HITRAN's and experimental parameters of the pressurebroadening of different lines of CO₂ having different quantum numbers.The graph shows that the pressure broadening coefficients are similarfor lines which belong to two different isotopes, but with the samequantum numbers.

FIGS. 3a and 3b illustrate exemplary CEOS systems according to differentembodiments.

FIG. 4 illustrates a method of performing an absorption measurementaccording to one embodiment.

FIG. 5 illustrates a method of performing an absorption measurementaccording to another embodiment.

FIG. 6 illustrates photo-acoustic signals taken at different pressures,and also a HITRAN comparison, for ¹³C and ¹²C isotopes of CO₂ accordingto an embodiment.

DETAILED DESCRIPTION

The present invention relates generally to trace gas detection and morespecifically to cavity enhanced absorption spectroscopy systems andmethods. Such systems and methods are useful for measuring the isotoperatio of trace gases and components of gas mixtures. Systems and methodsfor detecting trace gases according to various embodiments utilize aresonance optical cavity and a coherent light source coupled to thecavity, and provide improved accuracy and stability as compared toexisting systems and methods based upon similar principles.

FIG. 3a illustrates an exemplary cavity enhanced optical spectroscopy(CEOS) system 100 according to one embodiment. As shown, CEOS system 100includes a light source 101 that emits continuous wave coherent light,such as continuous wave laser light, an optical cavity 104 and adetector system configured to measure absorption within the cavity, andhence an absorption coefficient, as well as other characteristics ofincident and/or reflected light. In one embodiment, the detector systemincludes detector 110, however, the detector system may also includeoptional detectors 108 and/or 109. As shown, cavity 104 is a V-shapedcavity defined by cavity coupling mirror 105 and mirrors 106 and 107. Anoptional enclosure or housing (not shown) provides an air tight seal forcavity 104 such as to allow control of the environment within thehousing and hence the cavity 104. Enclosed cavities are desirable forcertain applications. One or more optical components (M) 102 areconfigured and arranged to facilitate directing, and mode matching laserlight from source 101 to the optical cavity 104 via cavity couplingmirror 105. In the embodiment shown in FIG. 3a , an optional beamsplitting element 103 is positioned and aligned so as to allowsubstantially all of the incident light 112 emitted or generated bysource 101 to impinge on cavity coupling mirror 105. A small portion ofthe incident light beam 112 is directed (e.g., reflected or refracted)by element 103 to optional detector 108, which can be used to measurethe light incident on the cavity. Cavity coupling mirror 105, in thisembodiment, is arranged at an angle with respect to beam 112 such that aportion of incident light 112 is reflected off of mirror 105 asreflected beam 114 and detected by optional detector 109. A portion ofincident light 112 enters cavity 104 via mirror 105. Depending on thefrequency of incident light 112 and the optical length of cavity 104(e.g., optical length from mirror 107 to mirror 105 to mirror 106),light 118 circulating in the cavity 104 may build up and resonate at oneor a plurality of cavity modes (cavity resonances evenly separated infrequency; commonly known as the FSR or free spectral range of thecavity). A small portion of the intracavity light 118 circulating incavity 104 between mirror 107, 105 and 106, emerges or escapes viamirror 107 and also mirrors 106 and 107 as determined by theirtransmissivity. The light escaping mirror 105 impinges on element 103,which allows a small portion 120 to pass back to source 101, e.g., foroptical feedback. In certain aspects, light returning to source 101passes through optional phase control and/or attenuation elements 120,which advantageously provides for phase and/or intensity control of theoptical feedback provided to source 101 from cavity 104. Examples ofuseful elements 120 might include an electro-optic modulator thatimposes a modulation on the phase of the light and an attenuationelement such as a Faraday rotator.

In certain embodiments, system 100 also includes a temperature sensorpositioned and configured to measure a temperature of the gas withincavity 104 and a pressure sensor positioned and configured to measure apressure of the gas within cavity 104. It should be appreciated thatmore than one temperature sensor may be used, and that more than onepressure sensor may be used. For example, a single temperature sensormay be used to determine a temperature internal to the cavity, or wheregas is flowed through the cavity, for example, two temperature sensorsmay be used to determine a temperature at a gas inflow port and a gasexhaust port, from which a temperature of the gas in the cavity can bedetermined. In certain embodiments, particularly closed cell or closedcavity embodiments, the temperature and pressure of the gas in thecavity is controlled using a temperature control element and a pressurecontrol element. Control of the ambient conditions, e.g., temperatureand/or pressure, can be useful to help improve signal resolution andSNR. For example, FIG. 6 illustrates photo-acoustic signals taken atdifferent pressures, and also a HITRAN comparison, for ¹³C and ¹²Cisotopes of CO₂.

In certain aspects, source 101 includes a laser or other coherent lightsource that is sensitive or responsive to optical feedback and thatemits radiation at the desired wavelength(s) or desired wavelengthrange(s). One useful laser is a semiconductor diode laser that issensitive to optical feedback from light impinging on the laser from thecavity coupling mirror 105. Other laser sources might include diodelasers, quantum cascade lasers and solid state lasers. Thereflectivities of mirrors 105, 106 and 107 define the optical feedbackintensity. U.S. patent application Ser. No. 13/252,915, filed Oct. 14,2011, which is incorporated herein by reference in its entirety,discloses laser based cavity enhanced spectroscopy systems includingmirror optimization techniques. It should be appreciated that the mirror105 through which the laser light enters the cavity has a powerreflectivity coefficient R₁ close to, but less than, unity such that thequantity T=1−R₁ is in the range from 10⁻¹ to 10⁻⁵. The other cavitymirror(s) should have a power reflectivity R₂ equal to or higher thanR₁. Such high reflective mirrors will certainly have some residualtransmission, even though it may be as low as a few or several ppm.

In certain aspects, source 101 is capable of being frequency scanned,whereby a mean optical frequency of the laser is adjustable or tunableover a range of frequencies. This can be accomplished as is well known,such as, for example, by adjusting the current applied to a diode laserand/or adjusting a temperature of the laser medium. In certain aspects,the cavity 104 is also capable of being frequency scanned, e.g., bychanging or adjusting an optical length of the cavity, whereby anoptical frequency of a cavity resonance peak is adjustable over a rangeof frequencies. Adjustment of the optical length of the cavity caninclude adjusting a relative position of one or more of the cavitymirrors (e.g., using a piezo element), and/or adjusting a pressure ofthe medium within cavity 104. An intelligence module or control module,such as a computer system, processor, ASIC or other control circuitry,is provided to enable automated control of the source frequency tuningor scanning and/or cavity optical length adjustment.

In certain embodiments, CEOS system 100 is useful for detecting isotopesor trace gases within a gas mixture present in the cavity 104. When thefrequency of the incident light 112 emitted by source 101 approaches thefrequency of one of the cavity modes, the incident light 112 enteringthe cavity 104 begins to fill the cavity to that mode and may lock tothat cavity mode. The optical intensity of the light 118 circulatinginside the resonance cavity reflects total cavity loss at the momentwhen the light frequency of incident light 112 coincides with the cavitymode transmission peak. The total cavity loss is a sum of the cavitymirror losses and losses caused by absorption by the medium present inthe cavity, e.g., absorption caused by absorbing analyte species presentin the gaseous or liquid medium in cavity 104. Examples of such speciesdetectable by embodiments herein include H₂O, N₂O, NO, NO₂, CO₂, CH₄,various hydrogen, carbon, nitrogen and oxygen isotopes, and many others.The isotopes may have close quantum numbers, e.g., dn=2, 0, +2, ordn=−2, −1, 0, +1, +2, or dn=−1, 0, +1, For carbon isotopes of CO₂, forexample, the lines are defined by even numbers, so the differencebetween two adjusted lines is +/−2.

In various embodiments, detector 110 is configured take measurementsfrom which an absorption coefficient can be determined, e.g., based onmeasuring the intracavity optical power with and without an absorbingspecies present. For example, the power circulating inside the cavity(P_(circ)) is determined by the equation P_(transm)=P_(circ)*T, where Tis the transmissivity of the mirror from which the light is escaping,and P_(transm) is the power detected by the detector. In FIG. 3,detector 110 is shown proximal to mirror element 107, however, it shouldbe appreciated that detection element 110 can be positioned to detectand measure the light escaping from mirror element 106 or mirror element105 (e.g., reflected off of the backside of beamsplitter (BS)). Also,detection element 110 could be configured and positioned internal to thecavity 104 to measure the intracavity optical power. In certainembodiments, each detector element (e.g., elements 109 and 110) includesa photodetector, such as a photodiode, and associated electronics, fordetecting light and outputting a signal representing the detected light.Examples of useful photodetectors might include silicon, InGaAs, Ge orGAP based photodetectors. Other useful detectors include CCDs,photomultipliers, etc. An intelligence module (e.g., a computer system,processor, ASIC or other control circuitry; not shown) receives thedetector output signals and processes these signals to produce orgenerate a signal that characterizes the cavity loss based on thedetection methodology used, e.g., PAS, free decay rate, phase shift,direct absorption, etc. For example, U.S. patent application Ser. No.13/218,359, filed Aug. 25, 2011, which is incorporated herein byreference in its entirety, discloses laser based cavity enhancedspectroscopy systems including techniques for producing normalizedsignals that are a linear function of total cavity loss and that are notsensitive to laser-cavity coupling.

Additionally, as mentioned above, other detection methods can be used,for example, cavity ring-down spectroscopy methods, or cavity enhancedphoto-acoustic spectroscopy (PAS) methods (see, e.g., U.S. patentapplication Ser. No. 12/660,614, (US Published Patent application2011-0214479 A1) filed on Mar. 2, 2010, entitled “METHOD AND APPARATUSFOR THE PHOTO-ACOUSTIC IDENTIFICATION AND QUANTIFICATION OF ANALYTESPECIES IN A GASEOUS OR LIQUID MEDIUM”, the contents of which are herebyincorporated by reference). For example, FIG. 3b shows system 100configured as a CE-PAS system including a photo-acoustic sensor 130according to one embodiment. Source 101 emits illumination at thedesired wavelength. The emitted radiation is mode matched to the cavity(defined by mirrors 104, 105 and 106) by mode matching optics 102 andenters the cavity via cavity coupling mirror 104. In one embodiment, thedetector system includes a detector 107 that measures the intensity ofintracavity optical power emerging from mirror 106 and a photo-acousticdetector 130, such as a quartz tuning fork or other photo-acoustictransducer, that measures photo-acoustic waves generated within thecavity. Measurements made by the detector system are used to determinean absorption coefficient for any gas species or isotopes present in thecavity. For CRDS measurements, the ring-down decay time is measured andused to determine the absorption coefficient.

Additionally, FIGS. 3a and 3b illustrate three mirror V-shaped cavities,however, it should be understood that the optical cavity could be alinear cavity including two or more mirrors, a ring cavity includingthree or more mirrors, or the cavity may take on any other configurationas may be apparent to one skilled in the art.

The methods described herein advantageously provide excellent accuracywith PAS methods in contrast to the common opinion of the reducedaccuracy of PAS. Usually photo-acoustic methods are known to give lessprecise information about the absorption coefficient, because the PASeffect depends on the presence in the gaseous sample of someuncontrolled components, such as for example moisture or other gases. Inthis case, the impact of the presence of other gases will be the samefor several isotopologues, and it will thus cancel out. Also, becausePAS is a zero baseline method, PAS may offer higher accuracy than othermethods. Moreover, even if the same or close quantum numbers are notused for different isotopologues, a PAS-based detection method willprovide good results, because the impact of the gas composition willstill be close for all isotopic species.

In certain embodiments, the frequencies of the cavity modes areadvantageously controlled so that specific gas/isotope absorption linesmatch up with cavity resonance peaks. In certain aspects, although theFSR is generally fixed, frequencies of the cavity modes are controlledby adjusting the optical cavity length. The optical cavity length can beadjusted by adjusting the cavity mechanical length, which can be done bymoving at least one of the cavity mirrors, or by changing the cavitybody temperature or by changing the cavity gas pressure.

FIG. 4 illustrates a method 400 of performing an absorption measurementaccording to one embodiment. The absorption measurement method 400 maybe performed using system 100 of FIG. 3 or other cavity system. In step410, a cavity mode is tuned to a desired wavelength. For example, inembodiments where two different isotopes, including different isotopeshaving close or equal quantum numbers, are being measured, the cavitymode is tuned to a first wavelength corresponding to a known absorptionline of first one of the two isotopes. Tuning the cavity mode in certainaspects includes adjusting a length of the cavity, e.g., by adjusting aposition of one or more mirrors defining the cavity, so that the cavityhas a resonance peak at the first wavelength. In step 420, light havingthe first wavelength is coupled with the cavity. For example, in certainaspects, the source (e.g., source 101) is adjusted or tuned to emitlight at the first wavelength, and the emitted light is coupled with orinjected into the cavity using mode matching optics as is well known. Instep 430, a first absorption signal is measured using a detector. Thedetector may include a photo-detector, a photoacoustic sensor, or otherdetector as described herein or another detector to measure theintracavity optical power at the corresponding wavelength. The firstabsorption signal gives information from which an absorption coefficientis derived. For example, the absorption signal may be proportional torepresentative of the absorption coefficient of an isotope or gasspecies at the first wavelength.

In step 440, the cavity mode is tuned to a different desired wavelength.For example, in embodiments where two different isotopes having close orequal quantum numbers are being measured, the cavity mode is tuned to asecond wavelength corresponding to a known absorption line of second oneof the two isotopes. Tuning the cavity mode in certain embodimentsincludes adjusting a length of the cavity, e.g., by adjusting a positionof one or more mirrors defining the cavity, so that the cavity has aresonance peak at the second wavelength. In step 450, light having thesecond wavelength is coupled with the cavity. For example, in certainaspects, the source (e.g., source 101) is tuned to emit light at thesecond wavelength, and the emitted light is coupled with or injectedinto the cavity using mode matching optics as is well known. In step460, a second absorption signal is measured using a detector. Thedetector may include a photo-detector, a photoacoustic sensor, or otherdetector as may be described herein to measure the intracavity opticalpower at the corresponding wavelength. The second absorption signalgives information from which an absorption coefficient is derived. Forexample, the absorption signal may be proportional to representative ofthe absorption coefficient of an isotope or gas species at the secondwavelength. In step 470, the first and second absorption signals areused to calculate the isotope ratio. Additional information such as gaspressure, gas composition, gas temperature and baseline absorption canalso be used in such calculations as is well known. For example, anintelligence module, such as a computer system, processor, ASIC or othercontrol circuitry, (not shown) receives the detector output signals andprocesses these signals to produce or generate the ratio, or tootherwise generate a signal that characterizes the cavity loss based onthe detection methodology used. In certain aspects, step 470 isperformed in real time, and in other aspects, step 470 is performed postdata acquisition. In step 480, the result of step 470 is output ordisplayed (e.g., rendered on a display device or printed on viewablemedia). Alternatively, or additionally, the data (e.g., absorption atfirst wavelength and absorption at second wavelength) is output ordisplayed.

FIG. 5 illustrates a method 500 of performing an absorption measurementaccording to another embodiment. The absorption measurement method 500may be performed using system 100 of FIG. 3 or other cavity system. Instep 510, a cavity length is selected or set such that specificabsorption line measurements for specific isotopes or gas species can bemade, e.g., based on predetermined or known absorption lines dependingon the desired gas species or isotopes to be measured. For example, inembodiments where two different isotopes are being measured, the cavitylength is tuned or adjusted such that the free spectral range of thecavity is equal to the difference in frequency between absorption linesof the two isotopes divided by an integer number. In embodiments, wheretwo different gas species are being measured, the cavity length is tunedor adjusted such that the free spectral range of the cavity is equal tothe difference in frequency between absorption lines of the two gasspecies divided by an integer number. Selecting the length of the cavityby design can be done by adjusting a position of one or more mirrorsdefining the cavity. In step 520, light having a first wavelengthcorresponding to an absorption line/wavelength of a first one of theisotopes or gas species is coupled with the cavity. For example, incertain aspects, the source (e.g., source 101) is tuned to emit light atthe first wavelength, and the emitted light is coupled with or injectedinto the cavity using mode matching optics as is well known. In step530, a first absorption signal is measured using a detector. Thedetector may include a photo-detector, a photoacoustic sensor, or otherdetector as may be described herein to measure the intracavity opticalpower at the corresponding wavelength. The first absorption signal givesinformation from which an absorption coefficient is derived. Forexample, the absorption signal may be proportional to or representativeof the absorption coefficient of the first isotope or gas species at thefirst wavelength.

In step 540, light having a second wavelength corresponding to anabsorption line/wavelength of a second one of the isotopes or gasspecies is coupled with the cavity. For example, in certain aspects, thesource (e.g., source 101) is tuned to emit light at the secondwavelength, and the emitted light is coupled with or injected into thecavity using mode matching optics as is well known. In step 550, asecond absorption signal is measured using a detector. The detector mayinclude a photo-detector, a photoacoustic sensor, or other detector asmay be described herein to measure the intracavity optical power at thecorresponding wavelength. The second absorption signal gives informationfrom which an absorption coefficient is derived. For example, theabsorption signal may be proportional to or representative of theabsorption coefficient of the second isotope or gas species at the firstwavelength. In step 560, for isotopes, the first and second absorptionsignals are used to calculate the isotope ratio. For gas species, thefirst and signals are used to derive the absorption coefficients and/orconcentration. Additional information such as gas pressure, gascomposition, gas temperature and baseline absorption can also be used insuch calculations as is well known. For example, an intelligence module,such as a computer system, processor, ASIC or other control circuitry,(not shown) receives the detector output signals and processes thesesignals to produce or generate the ratio, or to otherwise generate asignal that characterizes the cavity loss based on the detectionmethodology used. In certain aspects, step 560 is performed in realtime, and in other aspects, step 560 is performed post data acquisition.In step 570, the result of step 560 is output or displayed.Alternatively, or additionally, the data (e.g., absorption at firstwavelength and absorption at second wavelength) is output or displayed.

In certain aspects, the intelligence module such as a processor orcomputer system provides control signals to the various systemcomponents as necessary, and receives data and other signals from thevarious detectors and other components. It should be understood that theintelligence module could be a separate device or could be integratedwith a spectroscopic analysis or gas analyzer system. It should also beunderstood that the intelligence module may be configured to merelycollect and store the signals/data and that the collected signals/datamay be transmitted to, sent to, or otherwise provided to a separatesystem that implements the signal/data processing and computationfunctionality described herein.

In some embodiments, a baseline is defined or determined by measuring anabsorption coefficient at a wavelength that does not correspond with anabsorption line of any of the isotopes or gas species being measured oranalyzed. This is done in certain embodiments, by tuning the cavity to amode that is resonant at a wavelength away from an absorption line andinjecting an appropriate wavelength of light into the cavity. Thebaseline can be used in the methods 400 or 500 to produce a correctisotope ratio value, for example.

It should be appreciated that the various calculation and dataprocessing processes described herein may be implemented in processorexecutable code running on one or more processors. The code includesinstructions for controlling the processor(s) to implement variousaspects and steps of the gas analysis processes. The code is typicallystored on a hard disk, RAM or portable medium such as a CD, DVD, etc.The processor(s) may be implemented in a control module of aspectroscopic gas analysis system, or in a different component of thesystem such as gas analyzer having one or more processors executinginstructions stored in a memory unit coupled to the processor(s). Theprocessor(s) may be part of a separate system directly or indirectlycoupled with the gas measurement system. Code including suchinstructions may be downloaded to the system or gas analyzer memory unitover a network connection or direct connection to a code source or usinga portable, non-transitory computer-readable or processor-readablemedium as is well known.

One skilled in the art should appreciate that the processes of thepresent invention can be coded using any of a variety of programminglanguages such as C, C++, C#, Fortran, VisualBasic, etc., as well asapplications such as Mathematica® which provide pre-packaged routines,functions and procedures useful for data visualization and analysis.Another example of the latter is MATLAB®.

While the invention has been described by way of example and in terms ofthe specific embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements aswould be apparent to those skilled in the art. Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

What is claimed is:
 1. A gas analyzer system for measuring aconcentration of a component in a gas mixture, the system comprising: aresonant optical cavity having two or more mirrors and containing a gashaving a chemical species to be measured, wherein the resonant opticalcavity defines a plurality of cavity modes having a free spectral range(FSR); control circuitry configured to adjust or tune an optical pathlength of the resonant optical cavity so that a frequency of one of theplurality of cavity modes corresponds to an absorption frequency of thechemical species; a continuous-wave tunable laser optically coupled withthe resonant optical cavity, wherein an output of the laser is scannedacross a range of frequencies including the frequency of said one of theplurality of cavity modes; and a detector system for measuring anabsorption of laser light by the gas in the resonant optical cavity. 2.The system of claim 1, further including; a temperature sensor formeasuring a temperature of the gas in the resonant optical cavity; and apressure sensor for measuring a pressure of the gas in the resonantoptical cavity.
 3. The system of claim 1, wherein the detector systemincludes a photo-detector configured to measure an intensity ofintra-cavity light.
 4. The system of claim 1, wherein the absorptionfrequency of the chemical species corresponds to an absorption frequencyof an isotope of the chemical species.
 5. The system of claim 1, whereinthe resonant optical cavity is disposed in a housing that provides anairtight seal for the resonant optical cavity, and wherein thetemperature and the pressure of the gas in the housing are activelycontrolled.
 6. The system of claim 1, further comprising a temperaturecontrol element configured to control a temperature of the gas in theresonant optical cavity and a pressure control element configured tocontrol a pressure of the gas in the resonant optical cavity.
 7. Amethod of performing an absorption measurement in a cavity having two ormore cavity mirrors, wherein the cavity has a plurality of resonantcavity modes having a free spectral range (FSR), the method comprisingthe steps of: a) tuning the cavity so that a frequency of one of theplurality of cavity modes corresponds to an absorption frequency of afirst chemical species in a gas to be measured; b) coupling output lightof a continuous-wave tunable laser with the cavity; c) scanning theoutput light of the laser across a range of frequencies including thefrequency of said one of the plurality of cavity modes; d) measuring asignal corresponding to an absorption at the frequency of said one ofthe plurality of cavity modes; and e) calculating an absorptioncoefficient of the chemical species in the gas based on the measuredsignal.
 8. The method of claim 7, wherein tuning includes adjusting anoptical path length of the cavity.
 9. The method of claim 8, whereinadjusting an optical path length includes adjusting a relative positionof one of the cavity mirrors and/or adjusting a pressure of the gas inthe cavity and/or adjusting a temperature of the gas in the cavity. 10.The method of claim 7, wherein scanning includes adjusting a currentapplied to the laser and/or adjusting a temperature of a laser medium ofthe laser.
 11. The method of claim 7, further including calculating aconcentration of the chemical species based on the absorptioncoefficient.